The specific absorption of ions readily occurs at the interface between a non-conductive solid and an electrolyte solution. The exposed surface of any solid is covered with absorbed ions which define the limits of the inner Helmholtz plane. The accumulative charge making up this plane may be stoichiometrically compensated by an excess of oppositely charged ions diffusely dispersed throughout the bulk of a joining liquid phase in a direction perpendicular to the exposed solid surface. Ions of the opposite charge to those surface adsorbed can approach no closer than the outer Helmholtz plane. The space charge in the region between the outer immobile Helmholtz plane and the contacting mobile electrolyte gives rise to an electrokinetic driving force (defined as zeta potential) which plays an important role in explaining the equilibrium behavior of heterogeneous liquid-liquid or liquid-solid systems in industrial and civic processes such as flocculation control, water purification, waste management, etc. The influence of this electrical double layer upon equilibrium has been recognized for many years, however, any practical application of the concept has been seriously hampered by the absence of a suitable means of measurement.
It is conventional practice in clarifying aqueous systems containing suspended particles to employ a flocculation process. Once flocculated, the suspended particles can be separated from their fluid medium by sedimentation, filtration, floatation, centrifugation or one or more of the foregoing physical separatory processes in combination. Conventionally, the flocculation process is promoted by the use of flocculating chemicals such as alum, ferric chloride or various polymeric materials such as water-soluble cationic and anionic organic polyelectrolytes. Aqueous suspensions of finely divided polymeric particles are encountered in a paper machine headbox within a paper manufacturing process operation and in many other contexts. In a typical flocculation process for flocculating aqueous suspensions of finely divided particles, a water-soluble cationic flocculating chemical is added to the solution. The finely divided particles suspended in the solution are normally negatively charged and thus, the addition of the cationic agent results in charge neutralization on the suspended particles. When the average charge is zero, or some other predetermined value, the dispersed organic and/or inorganic particles undergo flocculation, i.e., aggregation at an increased rate. Too much cationic agent, however, creates positively charged particles which can be as difficult to flocculate as are the originally negatively charged particles.
To date, however, determining how much chemical to add to the stream to be treated has been difficult, especially since the composition of such stream often varies over fairly wide ranges and time intervals of a few minutes to a few hours.
Various empirical approaches to "finding" the correct dosage of flocculate to be added to a stream have been used. For example, increasing amounts of flocculant may be added to samples from the stream and amount of decrease in turbidity of the stream noted, the correct dosage being determined as the one which causes the greatest decrease in turbidity with the least addition of flocculant. Such a procedure is time consuming and therefore not really suitable where the composition of the treated solution varies, since the information resulting from this procedure is no longer valid or applicable since the treated solution will have varied by the time the data has been corrected.
Another approach is to use a so-called Zeta meter to determine zeta-related electrokinetic phenomena such as the charge condition existing in the stream. The Zeta meter is used to observe the time required for a single charge particle from the stream to pass a predetermined distance along a liquid path while under the influence of an electric field at a given temperature. The method is time consuming and requires a technician to perform the test and to interpret test results before the stream is treated with greater, lesser, or the same amount of flocculant as had been used since the last previous Zeta meter test was made.
U.S. Pat. No. 3,368,145 to W. F. Gerdes discloses an apparatus which provides a continuous measurement of the charge density of the absorbed material taken from the process stream.
U.S. Pat. No. 4,297,640 to Z. J. Moore which is herein incorporated by reference discloses an apparatus for measuring the electric charge of non-conductive particles in a mildly conductive electrolyte which incorporates a grounded stationary electrode in a special location with respect to two sensing electrodes. The guard electrode while effective does not completely eliminate all potential galvanic interferences.
U.S. Pat. No. 4,446,435 to Canzoneri discloses an ultrasonic streaming current detector for developing on a continuous basis, an electrical signal which is a function of the charge condition existing in a stream containing charged particles.
U.S. Pat. No. 4,449,101 to Canzoneri et al relates to a jet wash apparatus for an ultrasonic streaming current detector with means for variably conveying a cleaning fluid into the detector.
U.S. Pat. No. 4,820,990 to Z. J. Moore relates to an electrodeless apparatus for determining a function of an electrical charge condition utilizing an electrostatic or inductive coupling. The apparatus has limited utility since it is essentially for use in pure liquid systems.